Semiconductor devices are found in nearly every piece of consumer and commercial electronics made today. Their wide-spanning uses include single discrete devices such as diodes and transistors, as well as integrated circuits which can include many millions of semiconductor devices interconnected on a single semiconductor substrate. The discovery of new materials for use in semiconductor device manufacturing, as well as the development of new semiconductor device manufacturing methods, continues to improve the efficiency of these devices, as well as to expand the already broad range of their practical application.
Light emitting diodes (LEDs) are one example of a semiconductor device widely used in consumer and commercial applications. LEDs contain several semiconductor materials, including up-doped semiconductor material, an n-doped semiconductor material, and a junction between the two materials. As in a normal diode, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. When a voltage is applied with the correct polarity to the semiconductor structure, the junction is forward-biased, and the charge-carriers, electrons and holes, flow into the junction. When an electron meets a hole as it moves out of the n-doped region and into the junction, it falls to a lower energy level, and releases energy in the form of emitted light. The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the junction. Sometimes the n- and p-doped semiconductor material can include multiple layers of different semiconductor materials. Sometimes an active layer is sandwiched between the n-doped semiconductor material and the p-doped semiconductor material, allowing further control over both the wavelength of the photons emitted (e.g. color) and the number of photons emitted (e.g. brightness) when electrons move through the junction. Active layers can themselves include several layers of various semiconductor materials, and sometimes can contain several light emitting layers. LEDs with active layers comprising more than one light emitting layer are commonly called either multi-well (MW) LEDs or multiple quantum well (MQW) LEDs. In contrast, LEDs having a single light emitting layer in the active layer are commonly called either double heterostructure (DH) LEDs, or single quantum well (SQW) LEDs.
In order to utilize a semiconductor structure as a semiconductor device, electricity must be able to get to the structure; e.g., one must be able to apply a voltage across the structure. Since electrical potential and corresponding electrical current is generally transferred through a metallic medium, a connection between the metallic medium and the semiconductor structure is necessary to enable the application of voltage to the structure. Contacts are regions of a semiconductor structure that have been prepared to act as connections between the semiconductor structure and a metallic medium. Contacts that have low resistance, that are stable at various temperatures over time, and also that are stable when subjected to various electrical conditions over time, are critical for the performance and reliability of semiconductor devices. Other desirable properties include smooth surface morphology, simple manufacturing, high production yield, good corrosion resistance, and good adhesion to semiconductors. An ideal contact has no effect on the performance of the semiconductor structure, meaning that it has zero resistance and delivers the required current with no voltage drop between the semiconductor structure and the metal, and also meaning that the relationship between the voltage applied to the contact and the current generated in the structure is perfectly linear. In practice, a contact generally must have some resistance, but contacts that provide an approximately linear voltage-current relationship and that exhibit low resistance are desirable. These are referred to as ohmic contacts.
When two solids are placed in contact with one another, unless each solid has the same electrochemical potential, also called the work function, electrons will flow from one solid to the other until equilibrium is reached, forming a potential between the two solids, called the contact potential. A contact potential can give insulating properties to the connection between the two solids, and is the underlying cause of phenomena such as rectification in diodes. The contact potential causes the voltage-current relationship to be non-linear, and thus the connection between the two solids departs from ideal ohmic contact properties. In general, to create ohmic contacts with the lowest resistances and with the most linear and symmetric voltage-current relationship, materials with a work function near to the work function of the particular semiconductor material on which the ohmic contact is to be formed are sought.
Traditional methods of fabricating ohmic contacts on semiconductor structures, including structures that are to become LEDs, involve deposition of one or more various materials on the structure, such that the one or more materials only touch a specific part of the semiconductor structure. Generally, the materials as deposited on the semiconductor do not yet form an ohmic contact, because relationship between the work function of each material is such that undesirable contact potentials are formed. Therefore, the deposition step is followed by an annealing process to chemically alter the materials, which can correspondingly alter their work functions. During the anneal, diffusion of the atoms of the deposited layers and the contiguous portion of the semiconductor structure occurs, causing the materials to mix to varying degrees, essentially making the deposited layers part of the semiconductor structure while still allowing them to retain their basic physical shape. By allowing the relocation of atoms, annealing enables the formation of new chemical species with different properties than the originally deposited layers or the contiguous portion of the structure, and preferably results in the newly formed portion of the semiconductor structure having the desired ohmic contact properties. While annealing is generally essential for formation of an ohmic contact, high temperatures can introduce thermal defects into the semiconductor structure, leading to negative effects in the resulting semiconductor device, such as poor performance and poor operating lifespan. Additionally, high temperatures can cause undesirable changes in the surface characteristics (surface morphology) of the contact, such as beading and mottling, tending to make an electrical connection to the ohmic contact more difficult and less efficient. The negative effects of high temperature are compounded by a longer exposure to those temperatures. Therefore, compositions and methods for formation of ohmic contacts on semiconductor structures that can handle shorter anneals and that don't require high temperature anneals are sought.
For example, a common semiconductor used in LED semiconductor devices, and in other semiconductor devices, is gallium nitride (GaN), frequently found as layers of n-doped and/or p-doped material in the semiconductor structure. An ohmic contact is often sought to be formed with a specific layer of GaN, for example n-doped GaN (n-GaN). A stable metal-n-GaN system is imperative for the achievement of n-GaN-containing semiconductor devices, including LEDs. Contacts made by depositing titanium (Ti) followed by aluminum (Al) on the semiconductor structure are the most popular in n-GaN-containing semiconductor devices (Ti/Al-bilayer). However, the Ti/Al-bilayer system is easily prone to converting to an undesirable high-resistance contact after thermal annealing at an intermediate temperature range. This could be due to the formation of an aluminum oxide (Al2O3) on the Al, leading to an increase in the contact resistance. This change can be due to the formation of titanium nitride (TiN) during the annealing process. The Ti/Al-bilayer system can convent to an ohmic contact and exhibit a specific contact resistance that can be about 10−5˜10−6 Ωcm2 when annealed at higher temperatures. However, annealing at high temperatures can cause degradation in semiconductor device performance and reliability because Al has a low melting point (˜660 degrees C.) and tends to ball up during annealing. Thus, the surface morphology of most Ti/Al-bilayer based contacts is quite rough. In addition, application of high temperature to the semiconductor structure introduces thermal defects, which also can cause degradation in the performance of the semiconductor device.